Two Connecticut medical associations filed a lawsuit Wednesday against UnitedHealthcare alleging the insurer is breaking its contractual obligation with doctors by dropping them from the physician network for Medicare Advantage patients.

The lawsuit was filed in U.S. District Court by the Fairfield County Medical Association and Hartford County Medical Association on behalf of members. UnitedHealthcare and its parent company, UnitedHealth Group of Minnetonka, Minn., are named as defendants.

Last month, UnitedHealthcare started sending letters informing thousands of Connecticut doctors that they would be dropped from the insurer's Medicare Advantage network starting Feb. 1, 2014. The insurer will not say how many doctors it has dropped, but the Fairfield County Medical Association said it is 810 primary care physicians and 1,440 specialists across the state.

"Although no reason was provided for this unilateral termination, United's motives are nonetheless clear: By terminating numerous physicians from the MA networks, United seeks to stem financial losses occasioned by reduced federal payments under the Affordable Care Act," lawyers for the medical association wrote in their legal complaint.

Lawyers for the medical associations argue that UnitedHealthcare isn't allowed to cut the physician network as an "amendment" to its contract. The contract says either party may terminate the agreement by providing 90 days written notice, but lawyers for the medical associations say UnitedHealthcare's notice was deficient for various reasons.

The medical associations are asking for a permanent injunction that would allow doctors to continue to be in UnitedHealthcare's network. The medical associations also are asking for attorneys' fees and other related legal costs.

UnitedHealthcare spokeswoman Maria Gordon Shydlo said in a prepared statement, "We are focusing our Medicare Advantage network around the needs of our members to encourage higher quality health care coverage and help keep that coverage affordable. We will respond to the society's filing at the appropriate time."

The case has attracted the attention of Connecticut's congressional delegation, and all five House members and both U.S. senators wrote to UnitedHealthcare asking for information about cuts to the physician network. Connecticut's Attorney General George Jepsen wrote a letter Wednesday to a regional director of the U.S. Department of Health and Human Services imploring the agency to "aggressively scrutinize" the network cuts.

Jepsen said Wednesday the number of doctors cut from UnitedHealthcare's physician network is unprecedented in any context in Connecticut. His office doesn't have the authority to remedy the situation because Medicare Advantage is a federal program.

It's not entirely uncommon for a contract dispute between doctors and health insurers to become a court battle, though many times they are resolved by arbitration, said Matthew Katz, executive vice president of the Connecticut State Medical Society.

"It's not unusual for there to be legal action tied to an insurer and something they've done. What is unusual in this case with United is how many physicians have been terminated," Katz said.

Medicare is federal government-funded health insurance primarily for people 65 and older. Medicare Advantage is a version of Medicare Parts A and B, hospital and medical coverage, administered by private insurers. The insurers are paid by the federal government to provide coverage. Often, private insurers compete for market share by offering additional benefits, such as discounts on dental coverage, eyewear or hearing-aid services, in addition to exercise programs or gym memberships.